Carbureted engines have long been the most popular engine type used on personal and small utility aircraft. This is due to their simple design, reliability, and enhanced fuel-efficiency. Unique to the carbureted engine, however, is the need to alter the amount of fuel entering the engine's combustion chamber as altitude changes. In this blog, we will discuss how carbureted engines function, why there is a variable fuel requirement, and how the engine's design allows pilots to regulate the fuel mixture.
In a carbureted engine, atmospheric air is taken in through an intake located near the front of the engine. The air then flows through a narrowing tube and experiences the Venturi effect, which increases its velocity while decreasing the pressure. Simultaneously, fuel is pulled through a regulated fuel system before entering the mixing chamber. The fuel-air mixture then enters the intake manifold before being ignited. Finally, the exhaust from the combustion chamber is used to spin the propeller, inducing thrust.
When discussing any engine type, it is important to consider the conditions needed to reach optimal fuel efficiency. For carbureted engines, the correct fuel-to-air mixture changes with altitude. At sea level, where carburetors are usually calibrated, the fuel-mixture control is set to the full-rich position. A richer mixture leads to more fuel being brought from the fuel tank into the mixing chamber, whereas a leaner mixture implies the opposite. The problem is that as altitude increases, the air density decreases, meaning that the mass of air is reduced for the same volume. With less air per unit volume entering the chamber, the mixture of fuel-to-air will be oversaturated, causing a sharp decrease in fuel efficiency. Conversely, lean mixtures will burn with more efficiency, but cause the engine to run at a higher temperature, eventually damaging it.
The highest efficiency mixture possible is referred to as the ideal stoichiometric ratio. While one can get close to this ratio, in which all the oxygen and fuel entering the combustion chamber is consumed in equal amounts, it is nearly impossible to maintain. Additionally, many experienced pilots will purposefully alter the mixture for different stages of flight. For example, many prefer to taxi and take off with a rich mixture and lean the mixture out as they prepare to land.
Pilots control the fuel-air mixture by turning a small knob in the cockpit called the mixture needle. This knob controls a cable that connects to a valve in the fuel tank and acts as a floodgate. For example, if the pilot were to move the needle to the rich position, the cable would pull the valve away from the entrance of the nozzle, allowing large amounts of fuel to enter. The other mixture control system is the back-suction type, in which an adjustable valve controls the pressure in the fuel chamber. If set to the rich position, the valve will open and increase the pressure in the fuel tank, thus forcing as much fuel as possible into the nozzle.
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